The central goal of the Paris climate agreement is straightforward enough to quote it in whole:

Holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change.

But what difference does that 0.5°C make?

According to a paper published Monday (April 2) in the journal Nature Climate Change, it’s the difference between having sea ice or not in the Arctic during summers.

Through climate modeling, Alexandra Jahn, a climate scientist at the University of Colorado- Boulder, found that “constraining warming to 1.5°C rather than 2.0°C reduces the probability of summer ice-free conditions by 2100 from 100% to 30%.” In other words, letting world warm an extra half degree means increasing probability of having at least one ice-free Arctic summer by the end of the century by 70%.

Melting Ice in the Arctic

Image: National Snow and Ice Data Center

No more “old ice”

As we’ve noted before, one of the most important measures of climate change in the Arctic is how much “old ice”—thick, stable ice that manages to stay frozen over the summer months for at least four consecutive years—is left there. “As sea ice ages, it adds volume, expels salt, and is toughened up by jostling and collisions,” NOAA wrote in a 2017 report. “These characteristics make it better able to withstand warm weather and pounding from storm waves; its loss makes for a more fragile ice pack.”

But that old ice is declining as temperatures rise. It comprised 20% of the Arctic ice pack in 1985, but now, “sea ice more than four years old has nearly disappeared,” the NOAA researchers wrote.

Ice that can survive even one summer season is also becoming more scarce: Thicker ice more than one year old comprised 45% of the Arctic ice pack in 1985; in 2017, it was just 21%.

The implication of Jahn’s study is that by 2100, none of that old ice will survive the yearly summer thaw in a 2°C-warmed world.

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Will we make it?

The likelihood of actually meeting the Paris agreement goal is low. One study, published in 2017, found a 5% chance that countries would keep warming under 2°C. It put the chance of sticking to 1.5°C or less at merely 1%.

This finding could be viewed as one more reason to try.

“The good news is that sea ice has quick response times and could theoretically recover if we brought down global temperatures at any point in the future,” Jahn said in a press release. “In the meantime, though, other ecosystems could see permanent negative impacts from the ice loss, and those can’t necessarily bounce back.”